The present invention relates to a process for producing a composite resonator-type semiconductor laser device which can be easily manufactured and which enables the gap between the resonators to be precisely controlled.
FIGS. 1(a) and 1(b) shows the layer structure of a laser device (cleaved coupled cavity laser) in which two laser portions having different cavity lengths are integrated on a chip, which is disclosed, for example, in Scientific American, 1984, p. 148, wherein FIG. 1(a) is a front view and FIG. 1(b) is a section view along the line IVb--IVb thereof. In these drawings, reference numeral 1 denotes a first electrode, 2 denotes a current blocking layer, 3 denotes a cap layer, 4 denotes a first cladding layer, 5 denotes an active layer, 6 denotes a second cladding layer, 7 denotes a substrate, 8 denotes a second electrode, 21 denotes an air gap, 22 denotes a first laser portion, and 23 denotes a second laser portion. Further, an arrow represents a laser beam.
Next, operation of the laser device will be described below. If an electric current is allowed to flow between the first electrode 1 and the second electrode 8, the electrons and holes are confined in the active layer 5 and undergo recombination to emit light. With the diode-type semiconductor laser device, if a current greater than a threshold value is allowed to flow, there develops inversion distribution, and laser beams are oscillated having predetermined wavelengths (longitudinal mode) determined by the lengths of resonators. The mechanism is the same as an ordinary fabry-perot type laser device. With the laser device of FIGS. 1(a) and 1(b) in which two independent laser portions having different cavity lengths are arranged with their optical axes in alignment, however, different longitudinal oscillation modes are generally exhibited by the first laser portion 22 and the second laser portion 23. Among them, only those modes having the same wavelengths interfere with each other to oscillate. Generally, the number of modes can be reduced to one. As described above, the laser device of this type features a stabilized longitudinal mode.
In the foregoing was described the effect that is obtained when the two laser portions are oscillated. Here, however, if the current flowing into the first laser portion 22 is selected to be smaller than a threshold value, and is changed, the number or carriers injected changes making it possible to change the refractive index of the first laser portion 22. Namely, the effect obtained is the same as the case in which the cavity length of the laser device is changed, and the wavelength of oscillation of the second laser portion 23 can be tuned.
In the foregoing was mentioned the composite resonator-type semiconductor laser device (cleaved coupled cavity laser) which employs cleavage. There has also been proposed an interference-type laser device as disclosed in Denpa Shimbun, Dec. 14, 1984, in order to stabilize the longitudinal mode utilizing the former interference effect. The structure of this device is shown in FIGS. 2(a) and 2(b), wherein FIG. 2(a) is a front view and FIG. 2(b) is a section view along the line Vb--Vb thereof. In this laser device, two laser portions having different cavity lengths are formed by growing crystals on a grooved substrate.
The light and/or carriers are confined in the transverse direction (direction of line A--A in FIGS. 1(a) and 1(b) and FIGS. 2(a) and 2(b) ) by the buried structure in FIGS. 1(a) and 1(b) or by the current blocking layer relying upon a V-shaped groove in FIGS. 2(a) and 2(b).
According to the conventional composite resonator-type semiconductor laser device of FIGS. 1(a) and 1(b), the air gap 21 is formed by cleavage involving considerable difficulty. According to the conventional composite resonator-type semiconductor laser device of FIGS. 2(a) and 2(b), on the other hand, the crystals must be grown on the groove involving difficulty in regard to growing the crystals.